Non-volatile memory (NVM) is a memory device that retains content stored therein even when power is removed. EEPROM and flash memory are two commonly used non-volatile memory devices. In particular, flash memory has become widely used in electronic devices, especially portable electronic devices, because of its ability to provide data storage at low power levels. Modern day flash memory devices are typically implemented using a floating gate MOS transistor device as the memory cells. A floating gate MOS transistor device includes a floating gate that is formed between a control gate and the channel region (the substrate) of the MOS device and at least partially vertically aligned with the control gate. Charge storage on the floating gate determines the stored data state (“0” or “1”) of the memory cell.
In a non-volatile memory cell implemented using a floating gate MOS device, programming of the memory cell, or writing data to the memory cell, is accomplished by transferring charge carriers from the semiconductor substrate (the source or the drain) to the floating gate by tunneling through the thin gate oxide layer. Typically, a block of non-volatile memory cells is first erased by applying bias conditions to remove the charges stored on the floating gate. Then, the non-volatile memory cells can be written or programmed, usually one byte or word at a time, by applying the bias conditions opposite to the erase operation. Erase and programming operation of non-volatile memory devices require a relatively large voltage and current and erase and programming cycles can be slow.
Transconductance is a measure that relates the current through the output of a device to the voltage across the input of the device. In an MOS device, the transconductance (gm) measures the ability of the MOS device to amplify an input voltage signal (the gate voltage) to provide an output current signal (the drain current). For a Flash memory cell, the transconductance (gm) measures the change in the drain current of the floating gate MOS device, also referred to as the cell current Icell, to the change in gate voltage (VGS) over a small interval on the cell-current-versus-gate-voltage curve.
Transconductance is an important electrical parameter for flash memory devices having impacts on the reliability of the flash memory devices. It is often necessary to ensure a certain level of reliability for the flash memory devices, especially for flash memory devices used in automotive applications. To guarantee a certain life time for the flash memory devices, flash memory devices with low transconductance (low gm) memory cells are screened out, typically during the wafer sort process. Low gm memory cells are not desirable as the low gm memory cells can cause over-erase problem in normal gm memory cells. Over-erased memory cells may suffer from read failure after successive program-erase cycles, negatively impacting the reliability of the flash memory device.
While it is necessary to screen out low gm memory cells in order to guarantee the reliability of a flash memory, detecting low gm memory cells is challenging as it is difficult to distinguish a memory cell with low transconductance from a memory cell with a lower erase threshold voltage.